1. Field of the Invention
The present invention relates to a video endoscope system which obtains image data of a body cavity by picking up images of the inside of the body cavity formed from auto-fluorescence caused by its living tissues for a diagnosis thereof. The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-351157 (filed on Nov. 17, 2000), which is expressly incorporated herein by reference in its entirety.
2. Description of the Related Art
A video endoscope system which picks up image formed from auto-fluorescence caused by living tissue as an object of examination irradiated with excitation light such as ultraviolet light to output image data representative of the captured images has been utilized. It is known that diseased living tissue causes auto-fluorescence at a lower intensity than that of normal tissue. Therefore, by observing the images formed from the auto-fluorescence, an operator can diagnose a region where intensity of the fluorescence is low as a diseased part.
This type of a video endoscope system includes a light source unit for emitting excitation light, an illumination optical system for guiding the emitted excitation light to an object of examination, an objective lens for forming an image of the object, and a CCD for converting the image of the object into video signals. When irradiated with the excitation light guided through the illumination optical system, the object generates auto-fluorescence. Thereupon, the objective lens receives the fluorescence, as well as the excitation light reflected by the surface of the object. An optical filter to cut down the ultraviolet wavelength components is interposed between the objective lens and the CCD. The fluorescence transmitted through the objective lens passes through the filter, while the excitation light which transmitted through the objective lens is cut down by the optical filter. The fluorescence passed through the optical filter forms an image near the imaging plane of the CCD. Meanwhile, despite filtering by the optical filter, a small amount of the excitation light transmits through the optical filter to enter the CCD. By such a reason, the image data output from the video endoscope system contains not only a component of the image formed from the auto-fluorescence caused by the object but also a component of the image formed from the excitation light. Accordingly, when the video signals are transmitted to the monitor, the image formed from the excitation light are superposed on the image formed from the auto-fluorescence. In other words, the excitation light interferes with the image formed from the auto-fluorescence, so that the operator is unable to precisely understand the condition of the auto-fluorescence of the object.
In order to eliminate the adverse effects due to the excitation light from the fluorescence image, the excitation light which enters the CCD must further be reduced. For that purpose, the transmittance of the optical filter which is composed of a large number of films deposited on a substrate with respect to the excitation light could be further lowered, if the number of the deposited films is increased. However, an increase in the number of deposited films will lead to an increase in the stress resulting in the optical filter being more likely to crack. This will cause a decrease in the yields of the optical filter in its fabrication process. Thus, increasing the number of deposited films of the optical filter is not a practical idea.
An object of the present invention is to provide a video endoscope which can eliminate the influence caused by excitation light not only with an optical filter, and a video endoscope system equipped with such an endoscope.
To achieve the object, a video endoscope according to the present invention includes: an objective lens system having a lens surface provided with a coating for transmitting visible light while reducing excitation light for exciting a living tissue to cause auto-fluorescence; an image pickup device for converting an image of an object formed through the objective lens system into video signal; and an optical filter interposed between the objective lens system and the image pickup device for transmitting visible light while reducing the excitation light. Further, a video endoscope system according to the present invention includes the above described video endoscope, an excitation light irradiating unit which irradiates excitation light to the object, and a processor which generates and outputes fluorescence video data corresponding to the image of the object formed from the auto-fluorescence caused by the object, based on the video signal obtained by the image pickup device of the video endoscope.
When the object is irradiated with the excitation light, both the auto-fluorescence emitted from the living tissue of the object and the excitation light reflected by the surface of the object enter the objective lens. However, according to the present invention, the excitation light is cut down by the coating provided to the objective lens system. The cut down excitation light is further cut down by the optical filter, which allows only the auto-fluorescence to reach the image pickup device. This image pickup device converts the image formed solely from the auto-fluorescence of the object into video signals. For the purpose described above, the coating and the filter should preferably have total transmittance of 0.1% or lower for the excitation light.
The objective lens system may be substantially telecentric toward the image side, so that the filter and the image pickup device receive light which are substantially parallel to the optical axis of the objective lens system. The coating of the objective lens system should preferably be provided on a rearmost surface of the objective lens system. In this way, the coating and the filter receive light rays which are substantially parallel to the optical axis of the objective lens system. In other words, the incident angle of the light onto the coating and the filter can be reduced. Thus, the coating and the filter can perform as intended and intercept the excitation light.
In addition to the function of substantially shutting off the excitation light while transmitting visible light, the coating and the filter as a whole may also have a characteristic to cut off infrared light with a predetermined wavelength used for laser treatment. Thereby, even during laser treatment using an infrared light, the infrared light reflected by the object are intercepted by the coating and the filter after entering the objective lens. Consequently, no infrared light reach the image pickup device. Thus the image pickup device is free from the adverse effects caused by infrared light. Either one or both of the filter and the coating may has/have the characteristic to block infrared light.